1. Field of the Invention
This invention relates to a video data compression system, and more particularly to a compression encoding system for use in a video recording unit which sequentially switches video data obtained from a plurality of video cameras such as those used in a video surveillance system.
2. Description of the Related Art
Conventionally, a video surveillance system which records video from multiple video cameras (hereafter simply called cameras) onto a long-time recording time-lapse VTR has been used. This system uses a switcher to switch video from multiple cameras in a time-dividing manner.
Earlier patent disclosures dealing with this type of video surveillance systems are found in Japanese Patent Laid-Open Application No. Sho 64-65989 and Japanese Patent Laid-Open Application No. Hei 5-73312.
The system disclosed in Japanese Patent Laid-Open Application No. Sho 64-65989 synchronizes multiple cameras with the reference signal to eliminate the discontinuity of video signals at switching time.
It also synchronizes the VTR recording timing with the video signal switching signal to minimize unstable operation time. This results in a shorter video switching period, making high-density recording possible.
In addition, detecting at the time of playback, the camera identification signal superimposed on the video signal allows only the desired video to be selectively monitored.
The system disclosed in Japanese Patent Laid-Open Application No. Hei 5-73312 switches timely between a 2-system video selection means and a 2-system video signal synchronization means to enable a video surveillance system, which has cameras not under control of external synchronization signals or each controlled by its own synchronization method, to synchronize video signals for proper switching.
A system which switches multiple cameras in a time-division manner to combine their video signals into one mixed video signal is called a frame switcher. Many such systems are commercially available.
Today, digital video recording and playback systems which record and play back digital image data have become popular.
In general, image data to be recorded digitally is compressed to prevent the amount of data from increasing and therefore to allow the recording medium to be used for recording for a longer period of time.
Well-known image data compression techniques include JPEG (Joint Photographic Experts Group) and MPEG (Moving Pictures Experts Group).
Because these techniques are described in detail in many books, the following briefly gives the summary of their algorithms.
First, JPEG will be described
JPEG divides an image into multiple small blocks and converts each block into two-dimensional frequency components through DCT (Discrete Cosine Transform). It then reduces the amount of data through non-linear quantization and entropy encoding.
A high intra-frame correlation in the image means closer two-dimensional frequency components, thus increasing the encoding efficiency and reducing the amount of data while ensuring the picture image quality.
However, an image composed of fine patterns results in a lower intra-frame correlation and therefore decreases the compression efficiency. Thus, a compression ratio of up to 10:1 is required to ensure a reasonable JPEG image.
Next, MPEG will be described.
In addition to the technique used by JPEG described above, MPEG uses an inter-frame correlation to reduces the amount of data. More specifically, it finds changes between every two frames and performs DCT on the changes. This is called inter-frame predictive encoding.
For slow-moving video with little or no change between frames, MPEG produces a very small amount of data.
Even for fast-moving video, MPEG provides an extremely high predictive encoding efficiency because it detects motion vector between frames and motion compensation for them.
As a result, the image quality is not degraded at all even when the compression ratio is several scores to one and, therefore, a recording/playback system using the MPEG compression algorithm can record video data longer in time than JPEG.